| Nickel based high-temperature alloys are widely used in hot end components such as aviation engines and gas turbines due to their excellent high-temperature mechanical properties,oxidation resistance,and hot corrosion resistance.IN738 alloy belongs toγ’precipitation strengthened nickel based high-temperature alloys are widely used as load-bearing components in the hot end of gas turbines,such as blades and rotors,and have excellent mechanical properties at high temperatures.However,traditional processing methods such as casting and forging are used to prepareγ’precipitation strengthened nickel based high-temperature alloys have the characteristics of coarse dendritic structure,high porosity,micro element segregation,and long processing cycles,which greatly limit the industrial application of these alloys.With the continuous development of additive manufacturing(AM)technology,selective laser melting(SLM)technology has achieved direct manufacturing of complex nickel based high-temperature alloy parts in aerospace,petrochemical and other fields.The high temperature gradient(10~6-10~8 K/s),high solidification rate(≥0.01 m/s),high residual thermal stress accumulation,and non-equilibrium short-time metallurgical characteristics exhibited in the SLM forming process directly lead to the deposited alloy having different characteristics from the deposited alloy.At present,research on SLM forming of IN738 alloy is mainly focused on the areas of printing process optimization and cracking mechanism exploration,while some scholars focus on the influence of heat treatment regimes before and after forming on its serviceability.Since the rapid cooling process during SLM forming inhibits the diffusion of solute atoms and the formation of supersaturated matrix,while the process further promotes the precipitation of solute atoms during the subsequent reciprocal thermal cycle and post-heat treatment,for the precipitation-reinforced nickel-based high-temperature alloy prepared by additive manufacturing(AM)technology,the conventional heat treatment regimes are not able to meet the service requirements of the alloys,so the development of heat treatment regimes suitable for the forming process characteristics and performance requirements of these alloys has become a hot research topic.Since the properties of precipitation-reinforced nickel-based high temperature alloys are determined by the microstructure of the alloy and the characteristics of theγ’precipitation phase.To this end,this study addresses the service environment of IN738 alloy by sub-solution treatment of SLM formed IN738LC alloy with different cooling methods to obtainγ’precipitation phase specimens containing different sizes,morphologies and volume fractions,and discusses in detail theγ’precipitation phase evolution mechanism under different cooling conditions,focusing on the hot resistant corrosion performance of the alloy in the molten salt of 75%Na2SO4+25%Na Cl by mass fraction at 900℃ and the electrochemical corrosion performance in a 3.5%Na Cl solution by mass fraction at room temperature.The interaction between theγ’precipitation phase morphology,size distribution and area fraction and the alloy’s hot and electrochemical corrosion performance was elucidated,and the hot and electrochemical corrosion mechanisms were revealed.The research work will provide technical support for optimising the heat treatment process parameters of IN738LC alloy prepared by SLM technology to facilitate its application under practical working conditions.The specific findings are as follows:After sub-solution treatment of SLM formed IN738LC alloy,the quenching organization was significantly different by water cooling(WC),air cooling(AC)and furnace cooling(FC)to 800℃.Theγ’precipitation phase of the WC specimen showed a bimodal distribution,consisting of a large size primaryγ’precipitation phase and a small size secondaryγ’precipitation phase,which preserved the organization of the alloy after sub-solution treatment.The AC specimenγ’precipitation phase of the specimen showed a single model distribution,with the smallest size of theγ’precipitation phase but a uniform and dense distribution and the largest proportion of the area fraction;the distribution of theγ’precipitation phase of the FC specimen was the same as that of the WC specimen,but the primary and secondaryγ’precipitation phases had the largest size and the smallest proportion of the area fraction.After different cooling rates,the matrix grain size and internal residual stresses in different planes follow the pattern of XZ plane being larger than XY plane,and the same plane follows the pattern of AC>WC>FC.The different planar specimens show obvious anisotropy in terms of hot corrosion resistance,with the XY planar specimens having better hot corrosion resistance than the XZ planar specimens due to their smaller grain size and lowerγ’precipitation phase area fraction.For the same plane specimen,FC specimenγ’precipitation phase area fraction is small,the Al content required for the formation of internal oxide layer in theγmatrix is high,and at the same time the exposed area of theγmatrix is large,therefore,the thickness of the generated internal and external oxide layer is larger and the specimen has good hot corrosion resistance;AC specimenγ’precipitation phase area fraction is large,the Al content required for the formation of internal oxide layer in theγmatrix is small and the exposed area of theγmatrix is small,As a result,the inner and outer oxide layers are thin and the specimens have poor hot corrosion resistance.The corrosion resistance of the WC specimen is between that of the AC and FC specimens.The different planar specimens have significant electrochemical corrosion anisotropy.The XY plane specimens have a smaller grain size andγ’precipitation phase fraction,lower residual stress,a positive self-corrosion voltage(Ecorr)and a larger radius of capacitive arc resistance and a smaller self-corrosion current(Icorr),therefore have better electrochemical corrosion resistance than the XZ plane.As the specimens in the same plane also follow the above rule,the degree of galvanic corrosion formed with theγsubstrate is small,the specimens have a lighter degree of passivation film rupture and shallow pitting pits,so the corrosion resistance is FC>WC>AC in order. |
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